This invention relates to a process for the standardized preparation of thermoplastic polyurethane elastomers by the deactivation of reactive metal traces by phosphorus compounds.
Thermoplastic polyurethane elastomers ("TPU's") have long been known. TPU's have industrial significance by virtue of the combination of good mechanical properties with the known advantages of inexpensive thermoplastic processability. A wide range of variation of mechanical properties can be obtained by using different chemical synthesis components. A review of TPU's and their properties and applications can be found, for example, in Kunststoffe, 68 (1978), pages 819 to 825, or Kautschuk, Gummi, Kunststoffe, 35 (1982), pages 568 to 584.
TPU's are prepared from linear polyhydroxy compounds (generally polyesters or polyethers), organic diisocyanates, and short-chain diols. They may be prepared continuously or discontinuously. The best known production processes, which are also used on an industrial scale, are the so-called belt process and the extruder process.
In practice, however, the preparation of TPU's is complicated by a variable content of metal traces in the raw materials, especially in the linear polyhydroxy compounds. These metal traces either come from the production plant or are added as catalyst in the preparation of the polyhydroxy compounds. For example, the metals may be present as organic or inorganic iron, copper, titanium, tin, antimony, or lead compounds. They are often subjected to decomposition kinetics which lead to a variable content in the raw materials. These metal traces in the ppm range can cause unwanted secondary reactions and/or, because of their effect on TPU reactions, uncontrolled temperature increases during the production of TPU's that lead to variations in the properties of the end products.
For these reasons, it is advantageous to deactivate the metal traces before or during preparation of the TPU's, so that the resultant constant, relatively low reactivity of the raw materials allows controlled linear synthesis of the TPU's.
Certain compounds are already known as deactivators for metal traces in the raw materials used for polyurethane reactions. Such compounds are primarily complexing agents, such as .beta.-diketones (DD 155,620) and N-heterocycles (DD 195,136); acids, such as phosphoric acid (U.S. Pat. No. 3,794,592); and oxidizing agents, such as H.sub.2 O.sub.2 (DD 148,883).
The disadvantages of these compounds, which are used primarily in crosslinked polyurethane systems, reside on the one hand in their limited effectiveness in the special TPU raw materials and on the other hand in the effects which they have on the properties of the thermoplastically processable polyurethane final product (e.g., discoloration through oxidation and/or hydrolysis).
Accordingly, there remains a need to deactivate the metal traces in the raw materials in a simple and effective manner which does not effect the properties of the TPU's.